Cardiovascular disease is the leading cause of death in the United States. Systemic arterial hypertension (or simply hypertension) is a major risk factor for the development of cardiovascular disease. Aldosterone and desoxycorticosterone (DOC) are mineralocorticoids that cause hypertension through increased urinary sodium (Na) retention and hypokalemic metabolic alkalosis through greater urinary acidification. Mineralocorticoids affect distal nephron and collecting duct (CD) ion transport to achieve these effects. The CD expresses at least two isoforms of H,K-ATPases, HK1 and HK2, that participate in acid secretion by the CD. Our recent studies also show that chronic DOC pivalate (DOCP) administration stimulates renal Na retention, weight gain, and metabolic alkalosis that are dependent on the presence of H,K-ATPases. Specifically, mice that lack both H,KATPases (HK1,2 -/-) failed to develop significant Na retention or metabolic alkalosis in response to DOCP stimulation. Comparison of the response of the single HK1 -/- to the double HK1,2 -/- strongly suggests an important physiological role for the H,K-ATPase HK2 isoform in the response to DOCP. Therefore, our central hypotheses are that: A) the H,K-ATPase HK2 isoform is critical for the chronic action of mineralocorticoids to produce renal Na retention, cause hypokalemic metabolic alkalosis, and increase blood pressure (BP); and that B) dietary K loading abolishes or attenuates mineralocorticoid action on Na retention and electrolyte abnormalities, which are dependent on the induction of the H,K-ATPase HK2 isoform in the kidney. We propose the following specific aims: 1) to determine the role of the H,K-ATPase HK2 isoform in the ion transport response to chronic mineralocorticoid treatment; 2) to determine the role of H,K-ATPase HK2 isoform in the regulation of blood pressure and the contribution of K intake and hypokalemia to these changes; 3) to determine if the differences in WT and HK2 KO animals in response to DOCP administration reflect a renal phenotype. We will test the novel physiological hypothesis that the H,K-ATPase HK2 isoform has an important role in Na homeostasis, volume regulation and BP control. As such, we have developed an integrated approach that combines whole animal metabolic balance studies, in vitro microperfusion, state-ofthe- art radiotelemetry measurement of BP, and renal transplantation to examine the physiological significance of HK2 stimulation by mineralocorticoids.